Line baking sheets with parchment or silicone mats, and preheat oven to 350 degrees F.

Melt chocolate in microwave on high power in 30 second intervals, and set aside to cool.

In a small mixing bowl, whisk together flour, cocoa, baking powder, salt, and cinnamon.

In bowl of stand mixer, cream together granulated sugar and butter until light. Scrape bowl frequently. Add egg and vanilla, and beat another 2-3 minutes. Add cooled chocolate, and beat until fully combined. Using a spoon, fold in dry ingredients. Stir just until combined. Do not over-mix.

Roll dough into 1 ½ -inch balls, and place on prepared baking sheet. Bake at 350 degrees for 7-9 minutes.

While cookies are baking prepare frosting.

In 2-quart pan over low heat, melt butter. Add cocoa and milk (or cream), increase heat to medium high, and stirring constantly, bring to a boil.

Remove from heat, and whisk in powdered sugar and vanilla.

When cookies are done baking, allow to cool on pan for 2 minutes, and then remove to cooling racks. Immediately top each hot cookie with hot frosting, and sprinkle with chopped walnuts, if desired. Allow to cool completely for the frosting to set.

Store cool cookies in airtight container in the fridge